work on file encoding detection

[bcroq]

Here is a proposed patch for #842 that works on Python 2 and Python 3.
Tested with a UTF-8 encoded file and a latin-9 encoded file:

$ isort - < test_utf8.py && echo ok
# coding: utf-8

x = 'testé'
ok
$ isort - < test_latin9.py && echo ok
# coding: latin9

x = 'testé'
ok

$ isort test_utf8.py && echo ok
ok
$ isort test_latin9.py && echo ok
ok

Also tested with flake8-isort:

$ flake8 --no-isort-config test_utf8.py && echo ok
ok
$ flake8 --no-isort-config test_latin9.py && echo ok
ok

[timothycrosley]

This looks great, thank you!